Cancer is the second most common cause of death in the United States, exceeded only by heart disease. In the United States, cancer accounts for 1 of every 4 deaths. The 5-year relative survival rate for all cancer patients diagnosed in 1996-2003 is 66%, up from 50% in 1975-1977 (Cancer Facts & Figures American Cancer Society: Atlanta, Ga. (2008)). This improvement in survival reflects progress in diagnosing at an earlier stage and improvements in treatment. Discovering highly effective anticancer agents with low toxicity is a primary goal of cancer research.
Microtubules are cytoskeletal filaments consisting of αβ-tubulin heterodimers and are involved in a wide range of cellular functions, including shape maintenance, vesicle transport, cell motility, and division. Tubulin is the major structural component of the microtubules and a well verified target for a variety of highly successful anti-cancer drugs. Compounds that are able to interfere with microtubule-tubulin equilibrium in cells are effective in the treatment of cancers. Anticancer drugs like taxol and vinblastine that are able to interfere with microtubule-tubulin equilibrium in cells are extensively used in cancer chemotherapy. There are three major classes of antimitotic agents. Microtubule-stabilizing agents, which bind to fully formed microtubules and prevent the depolymerization of tubulin subunits, are represented by taxanes and epothilones. The other two classes of agents are microtubule-destabilizing agents, which bind to tubulin dimers and inhibit their polymerization into microtubules. Vina alkaloids such as vinblastine bind to the vinca site and represent one of these classes. Colchicine and colchicine-site binders interact at a distinct site on tubulin and define the third class of antimitotic agents.
Both the taxanes and vinca alkaloids are widely used to treat human cancers, while no colchicine-site binders are currently approved for cancer chemotherapy yet. However, colchicine binding agents like combretastatin A-4 (CA-4) and ABT-751 (FIG. 19), are now under clinical investigation as potential new chemotherapeutic agents (Luo, Y.; Hradil, V. P.; Frost, D. J.; Rosenberg, S. H.; Gordon, G. B.; Morgan, S. J.; Gagne, G. D.; Cox, B. F.; Tahir, S. K.; Fox, G. B., ABT-751, “A novel tubulin-binding agent, decreases tumor perfusion and disrupts tumor vasculature”. Anticancer Drugs 2009, 20(6), 483-92.; Mauer, A. M.; Cohen, E. E.; Ma, P. C.; Kozloff, M. F.; Schwartzberg, L.; Coates, A. I.; Qian, J.; Hagey, A. E.; Gordon, G. B., “A phase II study of ABT-751 in patients with advanced non-small cell lung cancer”. J Thorac Oncol 2008, 3(6), 631-6.; Rustin, G. J.; Shreeves, G.; Nathan, P. D.; Gaya, A.; Ganesan, T. S.; Wang, D.; Boxall, J.; Poupard, L.; Chaplin, D. J.; Stratford, M. R.; Balkissoon, J.; Zweifel, M., “A Phase Ib trial of CA4P (combretastatin A-4 phosphate), carboplatin, and paclitaxel in patients with advanced cancer”. Br J Cancer 2010, 102(9), 1355-60.).
Unfortunately, microtubule-interacting anticancer drugs in clinical use share two major problems, resistance and neurotoxicity. A common mechanism of multidrug resistance (MDR), namely ATP binding cassette (ABC) transporter protein-mediated drug efflux, limits their efficacy (Green, H.; Rosenberg, P.; Soderkvist, P.; Horvath, G.; Peterson, C., “beta-Tubulin mutations in ovarian cancer using single strand conformation analysis-risk of false positive results from paraffin embedded tissues”. Cancer Letters 2006, 236(1), 148-54.; Wang, Y.; Cabral, F., “Paclitaxel resistance in cells with reduced beta-tubulin”. Biochimica et Biophysica Acta, Molecular Cell Research 2005, 1744(2), 245-255.; Leslie, E. M.; Deeley, R. G.; Cole, S. P. C., “Multidrug resistance proteins: role of P-glycoprotein, MRP1, MRP2, and BCRP (ABCG2) in tissue defense”. Toxicology and Applied Pharmacology 2005, 204(3), 216-237.).
P-glycoproteins (P-gp, encoded by the MDR1 gene) are important members of the ABC superfamily. P-gp prevents the intracellular accumulation of many cancer drugs by increasing their efflux out of cancer cells, as well as contributing to hepatic, renal, or intestinal clearance pathways. Attempts to co-administer P-gp modulators or inhibitors to increase cellular availability by blocking the actions of P-gp have met with limited success (Gottesman, M. M.; Pastan, I., “The multidrug transporter, a double-edged sword”. J Biol Chem 1988, 263(25), 12163-6.; Fisher, G. A.; Sikic, B. I., “Clinical studies with modulators of multidrug resistance”. Hematology/Oncology Clinics of North America 1995, 9(2), 363-82).
The other major problem with taxanes, as with many biologically active natural products, is its lipophilicity and lack of solubility in aqueous systems. This leads to the use of emulsifiers like Cremophor EL and Tween 80 in clinical preparations. A number of biologic effects related to these drug formulation vehicles have been described, including acute hypersensitivity reactions and peripheral neuropathies (Hennenfent, K. L.; Govindan, R., “Novel formulations of taxanes: a review. Old wine in a new bottle?” Ann Oncol 2006, 17(5), 735-49.; ten Tije, A. J.; Verweij, J.; Loos, W. J.; Sparreboom, A., “Pharmacological effects of formulation vehicles: implications for cancer chemotherapy”. Clin Pharmacokinet 2003, 42(7), 665-85.).
Compared to compounds binding the paclitaxel- or vinca alkaloid binding site, colchicine-binding agents usually exhibit relatively simple structures. Thus providing a better opportunity for oral bioavailability via structural optimization to improve solubility and pharmacokinetic (PK) parameters. In addition, many of these drugs appear to circumvent P-gp-mediated MDR. Therefore, these novel colchicine binding site targeted compounds hold great promise as therapeutic agents, particularly since they have improved aqueous solubility and overcome P-gp mediated MDR.
Prostate cancer is one of the most frequently diagnosed noncutaneous cancers among men in the US and is the second most common cause of cancer deaths with over 180,000 new cases and almost 29,000 deaths expected this year. Patients with advanced prostate cancer undergo androgen deprivation therapy (ADT), typically either by luteinizing hormone releasing hormone (LHRH) agonists or by bilateral orchiectomy. Androgen deprivation therapy not only reduces testosterone, but estrogen levels are also lower since estrogen is derived from the aromatization of testosterone, which levels are depleted by ADT. Androgen deprivation therapy-induced estrogen deficiency causes significant side effects which include hot flushes, gynecomastia and mastalgia, bone loss, decreases in bone quality and strength, osteoporosis and life-threatening fractures, adverse lipid changes and higher cardiovascular disease and myocardial infarction, and depression and other mood changes.
Leuprolide acetate (Lupron®) is a synthetic nonapeptide analog of naturally occurring gonadotropin-releasing hormone (GnRH or LHRH). Leuprolide acetate is an LHRH superagonist that eventually suppresses LH secretion by the pituitary. Leuprolide acetate acts as a potent inhibitor of gonadotropin secretion, resulting in suppression of ovarian and testicular steroidogenesis. In humans, administration of leuprolide acetate results in an initial increase in circulating levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH), leading to a transient increase in levels of the gonadal steroids (testosterone and dihydrotestosterone in males, and estrone and estradiol in premenopausal females). However, continuous administration of leuprolide acetate results in decreased levels of LH and FSH. In males, testosterone is reduced to castrate levels (below 50 ng/dL). In premenopausal females, estrogens are reduced to postmenopausal levels. Testosterone is a known stimulus for cancerous cells of the prostate. Suppressing testosterone secretion or inhibiting the actions of testosterone is thus a necessary component of prostate cancer therapy. Leuprolide acetate can be used for LH suppression, which is the reduction and lowering of serum testosterone to castrate levels to treat prostate cancer.
Malignant melanoma is the most dangerous form of skin cancer, accounting for about 75% of skin cancer deaths. The incidence of melanoma is rising steadily in Western populations. The number of cases has doubled in the past 20 years. Around 160,000 new cases of melanoma are diagnosed worldwide each year, and it is more frequent in males and Caucasians. According to a WHO Report, about 48,000 melanoma-related deaths occur worldwide per year.
Currently there is no effective way to treat metastatic melanoma. It is highly resistant to current chemotherapy, radiotherapy, and immunotherapy. Metastatic melanoma has a very poor prognosis, with a median survival rate of 6 months and a 5-year survival rate of less than 5%. In the past 30 years, dacarbazine (DTIC) is the only FDA-approved drug for metastatic melanoma. However, it provides only less than 5% of complete remission in patients. In recent years, great efforts have been attempted in fighting metastatic melanoma. Neither combinations of DTIC with other chemotherapy drugs (e.g., cisplatin, vinblastine, and carmustine) nor adding interferon-α2b to DTIC have shown a survival advantage over DTIC treatment alone. Most recently, clinical trials with antibodies and vaccines to treat metastatic melanoma also failed to demonstrate satisfactory efficacy.
Melanoma cells have low levels of spontaneous apoptosis in vivo compared with other tumor cell types, and they are relatively resistant to drug-induced apoptosis in vitro. The natural role of melanocytes is to protect inner organs from UV light, a potent DNA damaging agent. Therefore, it is not surprising that melanoma cells may have special DNA damage repair systems and enhanced survival properties. Moreover, recent studies showed that, during melanoma progression, it acquired complex genetic alterations that led to hyperactivation of efflux pumps, detoxification enzymes, and a multifactorial alteration of survival and apoptotic pathways. All these have been proposed to mediate the multidrug-resistant (MDR) phenotype of melanoma. With the rapidly rising incidence of this disease and the high resistance to current therapeutic agents, developing more effective drugs for advanced melanoma and other cancer types that can effectively circumvent MDR will provide significant benefits to cancer patients.